Birth order influences BMI and insulin sensitivity

A study of 50 overweight middle-aged men by
researchers at the University of Auckland’s Liggins
Institute, suggests that first-borns may be more likely than
second-borns to have a greater body-mass index (BMI).

They
also found that first-borns had a lower sensitivity to the
hormone insulin. Both BMI and lower insulin sensitivity are
considered risk factors for type 2 diabetes, hypertension
and coronary heart disease.

The research, published in the
journal Scientific Reports, adds to our understanding of the
potential long-term health effects of birth order, although
larger studies that include sibling pairs are needed to
fully evaluate this link.

Some evidence suggests that
birth order may influence metabolism and body composition,
from infancy to early adulthood, but the potential impacts
in mid-life have not been explored previously.

The
research by Professor Wayne Cutfield and colleagues studied
50 overweight, but otherwise healthy men between the ages of
35 and 55, who were recruited as part of two clinical trials
at the Liggins Institute.

They report that first-borns
were on average 6.9 kg heavier than second-borns and had a
greater BMI. Insulin sensitivity was also 33 percent lower
in first-born men than in second-borns, despite adjustment
for fat mass.

The research offers a broad assessment of
the possible effects of birth order on metabolic health. The
authors note, however, that the participant range was rather
narrow (overweight males living in a large urban centre),
and because sibling pairs were not studied, the study could
have underestimated the magnitude of birth order effects on
insulin sensitivity and other metabolic outcomes.

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